A music CD designed to be unplayable in a computer's CD-ROM drive has been distributed in the UK - the aim is to confound those who pirate music on the internet. Promotional copies of Michael Jackson's forthcoming single "You Rock My World" features a copy-prevention mechanism called key2audio. This causes a computer's CD-ROM drive to incorrectly interpret the stored data.

A Sony spokesman said: "There are no plans currently to use similar technology on commercial releases of this record." The copy protection on the promotional CD is designed to stop the single leaking onto the internet prior to its official release, he says. This has happened before with entire albums from other artists. However, some observers believe that Sony's action is just one part of a far-reaching strategy designed to stop listeners making digital copies of the music stored on CDs. They fear this will stop them making copies for their personal use.

Key2audio alters an audio CD during master copying to make it incompatible with CD-ROM, CD-R and CD-RW drives. A computer's CD-ROM makes use of a "table of contents" included on software CD's to help with error correction. It is possible to write to this area on an audio CD so that the CD-ROM misinterprets the data and it refuses to play. Sony says that key2audio is just one of several copy protection techniques it is looking at. More subtle copy protection systems have reportedly featured already on CDs commercially released by Sony and other companies. These are designed to play in CD-ROM drives but prevent music being copied to a computer's hard drive.

In these systems special errors are added to the data on an audio CD. Stand-alone CD players and CD-ROMs will correct automatically when playing music. But when the data is copied to a computer's hard drive, these errors are included and cause irrevocable damage.

The potential problem is that these CDs may not play on some CD players and may deteriorate more quickly. "Their life will be less," says Midgley. "The result will be that interference appears more quickly." Midgley also points out that only one computer programmer needs to upload a music file to the net for a copying spree to start. And the chances of at least one programmer breaking a copyright protection system is high, he says..."